Walker also said Cruz was the “best positioned by far to win the nomination and defeat Hillary Clinton in the fall.”

“He stays firm, he sticks to his guns, he’s been pushed many times over not only by the left, but sometimes by his own party but he’s stayed true to what he believes,” Walker said of Cruz.

Walker’s endorsement comes before the Badger State’s Republican primary April 5, a crucial contest in which Donald Trump needs a substantial haul of the state’s 42 delegates.

Walker, who has been critical of Trump, said today, “I wanted to make sure I was supporting someone, not against something or against someone.”

Trump tweeted Monday afternoon, “After the way I beat Gov. Scott Walker (and Jeb, Rand, Marco and all others) in the Presidential Primaries, no way he would ever endorse me!”

The announcement marks the latest in a string of anti-Trump endorsements from prominent Republicans. Former 2012 presidential nominee Mitt Romney backed both Cruz and Ohio Gov. John Kasich several weeks ago, while Jeb Bush and Lindsey Graham recently endorsed Cruz, as well.

The Wisconsin governor dropped out of the 2016 presidential race in September, after a mere 71 days of campaigning. In his speech announcing the suspension of his campaign, Walker encouraged other Republican candidates to drop out at the time when the Republican field had 16 contenders (Rick Perry dropped out before Walker).

"I encourage other Republican presidential candidates to consider doing the same, so that the voters can focus on a limited number of candidates who can offer a positive, conservative alternative to the current front-runner," Walker said in September, referring to Trump.

In an interview on “This Week” in November 2013, Walker said that while Cruz is a good guy, the 2016 Republican nominee needs to be “somebody who is viewed as being exceptionally removed from Washington.”

“I think it's got to be an outsider,” Walker told ABC News’ Jonathan Karl. “I think both the presidential, vice presidential nominees should either be a former or current governor, people who have done successful things in their states, who have taken on big reforms, who are ready to move America forward.”